1. Technical Field
The present disclosure relates to the fabrication of heterojunction transistors for high-frequency, high-power applications.
2. Description of the Related Art
A high electron mobility transistor (HEMT) is type of field effect transistor (FET) in which an electron current flows freely within a conduction channel in an un-doped semiconductor. Such a substantially unobstructed conduction channel forms adjacent to a heterojunction, i.e., a boundary between two different semiconductors. FIG. 1 shows an example of a conventional HEMT device 100 that includes a heterostructure 102 made of two different semiconductor materials, layers 102a and 102b [“High Electron Mobility Transistors,” Laboratory for Millimeter Wave Electronics, Zurich, Switzerland (http://www.mwe.ee.ethz.ch/en/about-mwe-group/research/vision-and-aim/high-electron-mobility-transistors-hemt.html)]. The conventional HEMT device 100 can be built on any one of various different semiconductor substrates 104. A buffer layer 106 can be inserted between the substrate 104 and the heterostructure 102. Source and drain contacts 108 and 110, respectively, and a gate 112 are formed on the upper layer 102b of the heterostructure 102. The gate 112 modulates electron mobility within a conduction channel 114.
Formation of the conduction channel 114 at the heterojunction can be understood by considering energy levels within the heterostructure. A series of plots 116 to the right of the conventional HEMT device 100 show electron concentration 118 and energy levels 120 and 122 as a function of depth below a surface 124 of the heterostructure 102, along a spatial axis 126. The upper semiconductor in the heterostructure 102, layer 102b, is a negatively doped material having a wide energy band gap. The lower semiconductor in the heterostructure 102, layer 102a, is an un-doped material having a narrow energy band gap. [The term “band gap” refers to the difference between the energy of conduction band electrons (free electrons) and the energy of valence band electrons (atomically bound electrons) i.e., the amount of energy needed to liberate valence electrons from atoms in the semiconductor crystal.] Because the band gaps differ, the conduction band energies 120 and 122 of the materials do not coincide. Thus, when two such semiconductors are placed in contact with one another, their energy levels are discontinuous at the boundary. Such a discontinuity gives rise to a potential well 117 that develops at the boundary (heterojunction). The potential well 117 traps unbound donor electrons from the n-doped material at the surface of the un-doped material, resulting in a peak electron concentration 118 at the heterojunction. Such trapped donor electrons are sometimes referred to as a “two-dimensional electron gas.” The location of the potential well 117 thus defines the conduction channel 114 of the HEMT. Source and drain regions at either end of the conduction channel 114 can be negatively doped or un-doped, depending on the device.
Because the conduction channel 114 lacks dopant impurities, electron mobility within the conduction channel 114 of a HEMT is high compared to the electron mobility in conventional FET devices. Such a high electron mobility allows a large electron current to flow within the conduction channel 114, thereby increasing the speed of the device. A voltage applied to the gate 112 alters the conductivity within the conduction channel 114, thereby modulating the electron current. The ability to support such a high electron current makes HEMT devices suitable for high-power, high-frequency applications such as chips used in RF communication devices (e.g., cell phones, satellite TV receivers, radar equipment, and the like). Furthermore, semiconductor materials typically used in HEMT devices include compound semiconductors that have high intrinsic electron mobility, such as, for example, gallium arsenide (GaAs), aluminum gallium arsenide (AlGaAs), gallium nitride (GaN), and indium gallium nitride (InGaN), among others. GaN HEMTs are known to perform particularly well in high-power applications.
Graphene has drawn attention in recent years as a material for use in FETs due to its extraordinary properties, as shown in FIGS. 2A and 2B. Graphene is a monolayer of carbon graphite atoms arranged in a honeycomb crystal lattice (FIG. 2A). Crystalline graphite is made of stacked sheets of graphene. Although graphene was known for many years, a single graphene sheet was not isolated until 2004, for which a Nobel prize in physics was awarded in 2010. Mechanically, graphene is one of the strongest materials ever tested, more than 100 times stronger than a comparable sheet of steel, (if steel could be made as thin as a graphene sheet). Graphene sheets are flexible and can be rolled into carbon nanotubes or formed into fullerene structures. Graphene is also very light weight, weighing only 0.77 mg per square meter.
Electrically, graphene has high electron mobility over a wide temperature range, lower resistance at room temperature than any known material, and low noise. Furthermore, a graphene film can be epitaxially grown on silicon carbide (SiC) by heating the SiC in a vacuum chamber to temperatures exceeding 1100C. The graphene film can then be patterned using conventional microelectronics techniques. Graphene has been studied as a material for use in microelectronics, such as in graphene field effect transistors (GFETs) [Meric, et al., Proceedings of the IEEE IEDM Conference, Dec. 5-7, 2011, pp. 2.1.1-2.1.4]. In a GFET, a graphene layer is used as the conducting channel to increase electron mobility for high-frequency applications. A GFET thus provides an alternative way to achieve a high electron mobility transistor without the use of a heterostructure.